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Detailed Fingerprints of Global Sea-level Change Revealed in Upper Devonian / Mississippian Woodford Shale of South-Central Oklahoma*

By

Stanley T. Paxton1, Anna M. Cruse2, and Alischa M. Krystyniak2

 

Search and Discovery Article #40211 (2006)

Posted September 21, 2006

 

*Oral presentation at AAPG Annual Convention, Houston, Texas, April 9-12, 2006.

 

Click to view presentation in PDF format (~9.3 mb).

 

1USGS Water Science Center, Oklahoma City, Oklahoma ([email protected])

2Oklahoma State University, Stillwater, Oklahoma

 

Abstract 

Gamma-ray logging profiles in organic-rich shale commonly contain intervals in which readings exceed 150 API units. These intervals of elevated gamma-ray, or “hot streaks”, are considered stratigraphic condensed sections; relatively long periods of continuous geological time amalgamated in and represented by a relatively a thin slice of stratigraphic section. Inspection of these profiles, however, reveals that “hot streaks” contain much more stratigraphic information than heretofore recognized. We have prepared full-scale displays of gamma-ray profiles from subsurface well logs through the marine Woodford Shale and made comparisons with spectral gamma-ray profiles collected at outcrop with a handheld spectrometer. Most of the Woodford has gamma-ray readings >150 API units. Some intervals of the formation contain up to 110 ppm uranium and approach 1000 API units. Our analyses indicate that the gamma-ray response in the Woodford is clearly dictated by the uranium in the shale. Noted by past researchers, and verified by our work, the distribution and intimate association of uranium with the shale suggests the source of the uranium was seawater. Therefore, the uranium enrichment was likely caused by diffusion from Devonian/Mississippian seawater into the upper 10-20 cm of the underlying, chemically-reducing seafloor mud. Once in the mud, uranium would have been fixed in the sediments through chemical reduction, thereby establishing a concentration gradient in the porewaters to further drive diffusion. In this sense, the concentration of uranium in the muddy substrate would be linked to sedimentation rate, with extremely slow or starved sedimentation exhibiting the most abundant uranium concentrations.

 

Selected Figures 

Distribution of Upper Devonian and Lower Mississippian black shales and related rocks in the United States (Conant and Swanson, 1961).

Location map of study area in southern Oklahoma.

Late Devonian paleogeography, North America (Blakey, 2005), with tectonic elements in Kansas and Oklahoma (Lambert, 1993).

Woodford Shale at Henryhouse Creek section.

Correlation of Woodford Shale from outcrop (Henryhouse Creek) to subsurface (Amis 1-3 Ramsey, 3-2S-2E, Murray County).

Correlation of gamma ray log of Woodford Shale in Amis 1-3 Ramsey to eustasy curve of Johnson, Klapper, and Sandberg (1985).

Gamma ray log of Woodford Shale, Henryhouse Creek, and the uranium component.

 

References 

Blakey, Ron, 2005, Paleogeogaphy and geologic evolution of North America: http://jan.ucc.nau.edu/rcb7/nam.html.

Conant, L.C., and Swanson, V.E., 1961, Chattanooga Shale and related rocks of central Tennessee and nearby areas: U.S. Geol. Survey Prof. Paper 357, 91 p.

Johnson, J. G., Klapper, Gilbert, and Sandberg, C. A., 1985, Devonian eustatic fluctuations in Euramerica, Geological Society of America Bulletin, v. 96, no. 5, p. 567-587.

Lambert, M.W., 1993, Internal stratigraphy and organic facies of the Devonian-Mississippian Chattanooga (Woodford) Shale in Oklahoma and Kansas, in B.J. Katz and L.M. Pratt, eds., Source rocks in a sequence stratigraphic framework: AAPG Studies in Geology 37, p. 163-176.